Video Transcript
When touched, mimosa leaflets close
up. What is a possible evolutionary
advantage of this? (A) The surface area for light to
be captured on is increased. (B) It makes the plant look wilted
and deters herbivores from eating it. (C) More carbon dioxide is taken in
while the plant is not photosynthesizing. Or (D) it prevents the entry of
pathogens and parasites.
The mimosa plant has a distinct
response to stimuli, such as being touched, or even to the absence of light. It closes its leaflets and
droops. The nondirectional movement of a
plant in response to touch is called thigmonasty, as the prefix thigmo- means
touch.
Many scientists have proposed
theories to try and explain why this response has evolved in the mimosa plant. To answer this question, we need to
work out which of the answer options is describing one of these possible
evolutionary advantages.
Answer choice (A) suggests that
when mimosa leaflets close up, the surface area that can capture light
increases. This is incorrect, as closing up
the leaflets actually decreases the surface area of the leaf and therefore its
photosynthesizing cells exposed to light.
Answer choice (C) suggests that
more carbon dioxide being taken in while the plant is not photosynthesizing is an
evolutionary advantage of the mimosa leaflets closing up. Carbon dioxide is actually one of
the key reactants that is required for photosynthesis. When the plant is not
photosynthesizing, it will not usually be taking in any carbon dioxide. So this option is also
incorrect.
Option (D) suggests that it might
be a mechanism to prevent the entry of pathogens and parasites. While plants do have many
adaptations to aid this, closing up the leaflets would not stop a pathogen from
entering it. So this option is also
incorrect.
When the mimosa plant closes up its
leaflets and droops slightly, it looks smaller and more wilted and theoretically
less appealing to a potential herbivore. This could explain a possible
advantage of this thigmonastic response to touch, as it could discourage nearby
herbivores from consuming the mimosa plant and as a result killing it. This means that we have found the
correct answer to this question.
A possible evolutionary advantage
of mimosa closing its leaflets in response to touch is (B). It makes the plant look wilted and
deters herbivores from eating it.
